This invention relates to channel plates which comprise a dielectric substrate having a set of elongate channels therein and dielectric material extending over the surface of the substrate to cover the channels and which are suitable for use particularly, but not exclusively, in flat display devices such as plasma addressed electro-optic display devices, and to methods of making such.
A known plasma addressed electro-optic display device using liquid crystal material and commonly referred to as a PALC display device typically comprises a sandwich of a first substrate having deposited on it parallel transparent column electrodes of ITO material and a colour filter layer; a second substrate comprising a channel plate having a set of parallel, sealed, plasma channels corresponding to rows of the display crossing all of the ITO columns and each of which is filled with a low pressure ionizable gas, such as helium, neon and/or argon, and containing spaced cathode and anode electrodes along the channel for ionizing the gas to create a plasma, which channels are closed off by a thin transparent dielectric sheet; and a liquid crystal (LC) material located between the substrates. The structure behaves like an active matrix liquid crystal display in which the thin film transistor switches at each pixel are replaced by a plasma channel acting as a row switch and capable of selectively addressing a row of LC pixel elements. In operation, successive lines of data signals representing an image to be displayed are sampled at column positions and the sampled data voltages are respectively applied to the ITO columns. All but one of the row plasma channels are in the de-ionized or non-conducting state. The plasma of the one ionized selected channel is conducting and, in effect, establishes a reference potential on the adjacent side of a row of pixels of the LC layer, causing each LC pixel to charge up to the applied column potential of the data signal. The ionized channel is turned off, isolating the LC pixel charge and storing the data voltage for a frame period. When the next row of data appears on the ITO columns, only the succeeding plasma channel row is ionized to store the data voltages in the succeeding row of LC pixels, and so on. As is well known, the attenuation of each LC pixel to backlight or incident light is a function of the stored voltage across the pixel. A more detailed description is Buzak et al., xe2x80x9cA 16-inch Full Colour Plasma Addressed Liquid Crystal Displayxe2x80x9d, Digest of Tech. Papers, 1993 SID Int. Symp., Soc. for Info, Displ. pp. 883-886.
A cross-section of the PALC display device described in the 1993 SID Digest is shown in FIG. 2. The thin dielectric sheet, referred to as the xe2x80x9cmicro-sheetxe2x80x9d, can be given a thickness in the range of 30-80 xcexcm. From the mechanical point of view this makes the sheet rather fragile and difficult to handle in a production environment and special precautions have to be taken in the fabrication process of the panels. Breakage of the micro-sheet is a yield limiting factor. From the electrical point of view the micro-sheet is too thick because it has a large capacitance which necessitates high driving voltages.
There is proposed in U.S. Pat. No. 5,244,427 a channel plate for a PALC display device in which the conventional glass micro-sheet is dispensed with and a deposited layer of dielectric material is used instead to close and seal in gastight manner the channels. The proposed method of fabricating the plate involves etching or machining channels in a glass substrate which are subsequently filled with a heat-resisting high molecular material so as to form a flat surface. A film of transparent dielectric material such as glass or SiO2 is then deposited. Thereafter the material filling the channels is dissolved and removed, presumably via the ends of the channels, leaving the deposited dielectric film in place which then serves the same function as the conventional glass micro-sheet. However, problems can be expected with this proposed method, particularly when used for large sized channel plates intended for large area display panels, in view of the difficulty in extracting efficiently and effectively the filler material from these channels, which may be for example around 1 metre in length but only around 300 xcexcm in width.
It is an object of the present invention to provide an improved channel plate and method for making such which is suitable for use in panel display devices such as PALC display devices or other electronic array devices, utilising a similar form of channel plate.
It is another object of the present invention to provide a plasma addressed electro-optic display device which can be manufactured more reliably, with higher yield.
According to one aspect of the present invention there is provided a method of fabricating a channel plate comprising a dielectric substrate having a set of elongate channels therein covered by a layer of dielectric material, which method comprises providing a substrate having the set of channels formed therein, filling the channels with filler material, depositing a layer of dielectric material over the substrate so as to cover the filler material in the channels and removing the filler material from the channels, which is characterised in that after filling the channels with filler material a layer of porous dielectric material is deposited over the substrate, the filler material is removed from the channels at least partly through the deposited porous layer, and thereafter a layer of non-porous dielectric material is deposited over the layer of porous material so as to close the set of channels.
According to another aspect of the present invention there is provided a method of making a plasma addressed electro-optic display device comprising a layer of electro-optic material between a first substrate, having data electrodes, and a channel plate comprising a set of elongate plasma channels in a dielectric substrate which channels each comprise electrodes and an ionisable gas filling and are covered by a layer of dielectric material, which comprises the steps of fabricating the channel plate by the method according to the first aspect of the invention, filling the channels with an ionisable gas, assembling the channel plate and first substrate together with a space therebetween and introducing electro-optic material into said space.
According to a further aspect of the present invention a channel plate, suitable for use in a plasma addressed electro-optic display device, is provided which comprises a dielectric substrate having a set of elongate channels therein, a thin layer of deposited porous dielectric material extending over the surface of the substrate and covering the channels, and a thin layer of deposited non-porous dielectric material extending over the porous layer to seal off the channels.
The use of a porous dielectric layer deposited over the channels enables the filler material contained therein to be readily extracted more directly along the length of the channel so that problems caused by trying to remove the filler material from the ends of the channels as in the earlier proposed method are avoided. After forming the porous layer, the structure can be immersed in a bath of a suitably selective solvent or etchant which penetrates through the pores of this layer and dissolves the filler material, allowing the filler material to be removed readily through the pores and via the ends of the channels. Also, the presence of the porous layer in the finished channel plate means that some communication between adjacent channels in the plate is possible via this layer below the non-porous layer rather that the channels being individually sealed. Thus, after the channels are filled with an ionisable gas and during subsequent use, equalisation of gas pressure in the set of channels can occur. Also the gas can be introduced into just one channel rather than all channels separately.
The filler material preferably comprises a polymer. Preferably, a liquid polymer material is applied over the substrate, by spin coating or printing, and then heat treated to form a polymerised film, the heat treatment serving to promote reflow so as to planarise this layer. A plasma etching or ashing process can then be used to etch the polymer layer until substantially level with the top of the channels so as to facilitate the deposition of a planar and uniform thickness porous layer.
The channels may be formed in the surface of the substrate, preferably of glass, by any suitable technique such as etching or mechanical grinding. Alternatively the substrate with the channels may be formed from a plate of glass on whose surface strips of dielectric material may be provided to form channel walls, each channel being defined by the facing sides of an adjacent pair of strips and the intervening surface region of the glass plate.
The deposited porous dielectric layer and overlying non-porous dielectric layer can be of various materials which are capable of being deposited in the desired manner and which offer appropriate properties, for example dielectric constant, but silicon oxide, silicon nitride and tantalum dioxide are particularly preferred. When using such materials, the non-porous layer can be deposited by sputtering, chemical vapour deposition (CVD), plasma enhanced CVD (PECVD) or ECR (electron cyclotron resonance, a form of PECVD), at a relatively low temperature, for example around 200xc2x0 C. or lower, and the porous layer is preferably deposited by a similar process to a thickness of between preferably 50 to 500 nm. The thickness and type of each layer is selected having regard to the desired electrical characteristics i.e. capacitance and resistance, and structural characteristics, i.e. mechanical integrity. In the case of the plate being used in a PALC display device, the deposited dielectric material must serve to seal the channels, to contain the ionisable gas filling therein, and the region containing the liquid crystal material, and therefore the nature of the non-porous deposited layer must be such that it is impermeable to both the ionisable gas and liquid crystal. With this in mind, and taking into account also the electrical characteristics required from the combined dielectric layers when in use as an addressing structure, the overall thickness of the two, porous and non-porous, layers may be in the range of 0.5 to 20 xcexcm, which is considerably thinner than the conventionally used glass micro-sheet.
Although the invention is applicable especially to plasma-addressed electro-optic display devices it is envisaged that the channel plate could be used as an addressing structure in different kinds of flat display devices and other electronic array devices, such as memory devices, particularly when the addressing structure incorporates an ionisable gas.
Embodiments of channel plates, plasma-addressed electro-optic display devices incorporating such, and their methods of manufacture, in accordance with the present invention will now be described, by way of example, with reference to the accompanying drawings in which: